【正文】 oilbased metalworking fluid instead of a waterbased coolant because oil provides greater lubricity. But if a shop prefers using coolant, the fluid should include EP (extreme pressure) additives to increase lubricity and minimize foaming. “If you’ve got a lot of foam,” Davis noted, “the chips aren’t being pulled out the way they are supposed to be.” He added that another way to enhance a tool’s slipperiness while extending its life is with a coating, such as titanium aluminum nitride. TiAlN has a high hardness and is an effective coating for reducing heat’s impact when drilling difficulttomachine materials, like stainless steel. David Burton, general manager of Performance Micro Tool, Janesville, Wis., disagrees with the idea of coating microtools on the smaller end of the spectrum. “Coatings on tools below typically have a negative effect on every machining aspect, from the quality of the initial cut to tool life,” he said. That’s because coatings are not thin enough and negatively alter the rake and relief angles when applied to tiny tools. However, work continues on the development of thinner coatings, and Burton indicated that Performance Micro Tool, which produces microendmills and microrouters and resells microdrills, is working on a project with others to create a submicronthickness coating. “We’re probably 6 months to 1 year from testing it in the market,” Burton said. The microdrills Performance offers are basically circuitboard drills, which are also effective for cutting metal. All the tools are without throughcoolant capability. “I had a customer drill a dia. hole in stainless steel, and he was amazed he could do it with a circuitboard drill,” Burton noted, adding that pecking and running at a high spindle speed increase the drill’s effectiveness. The requirements for how fast microtools should rotate depend on the type of CNC machines a shop uses and the tool diameter, with higher speeds needed as the diameter decreases. (Note: The equation for cutting speed is sfm = tool diameter spindle （論文）外文翻譯 3 speed.) Although relatively low, 5,000 rpm has been used successfully by Burton’s customers. “We remend that our customers find the highest rpm at the lowest possible vibration—the sweet spot,” he said. In addition to minimizing vibration, a constant and adequate chip load is required to perate the workpiece while exerting low cutting forces and to allow the rake to remove the appropriate amount of material. If the drill takes too light of a chip load, the rake face wears quickly, being negative, and tool life suffers. This approach is often tempting when drilling with delicate tools. “If the customer decides he wants to baby the tool, he takes a lighter chip load,” Burton said, “and, typically, the cutting edge wears much quicker and creates a radius where the land of that radius is wider than the chip being cut. He ends up using it as a grinding tool, trying to bump material away.” For tools larger than , Burton considers a chip load under to be “babying.” If the drill doesn’t snap, premature wear can result in abysmal tool life. Too much runout can also be destructive, but how much is debatable. Burton pointed out that Performance purposely designed a machine to have TIR to conduct inhouse, worstcase milling scenarios, adding that the pany is still able to mill a wide slot “day in and day out.” He added: “You would think with runout and a chip load a third that, say, to , the tool would break immediately because one flute would be taking the entire load and then the back end of the flute would be rubbing. When drilling, he indicated that up to TIR should be acceptable because once the drill is inside the hole, the cutting edges on the end of the drill continue cutting while the noncutting lands on the OD guide the tool in the same direction. Minimizing run out bees more critical as the depthtodiameter ratio increases. This is because the flutes are not able to absorb as much deflection as they bee more engaged in the workpiece. Ultimately, too much runout causes the tool shank to orbit around the tool’s center while the tool tip is held steady, creating a stress point wh